This disclosure relates to an electrochemical method, more particularly, relates to an oxidation and reduction process and even more particularly, relates to a process for producing chlorine dioxide.
With the decline of gaseous chlorine as a microbiocide and bleaching agent, various alternatives have been explored, including bleach, bleach with bromide, bromo-chlorodimethyl hydantoin, ozone, and chlorine dioxide (ClO2). Of these, chlorine dioxide has generated a great deal of interest for control of microbiological growth in a number of different industries, including the dairy industry, the food and beverage industry, the pulp and paper industries, the fruit and vegetable processing industries, various canning plants, the poultry industry, the beef processing industry and miscellaneous other food processing applications. Chlorine dioxide is also seeing increased use in municipal potable water treatment facilities, potable water pathogen control in office building and healthcare facilities, industrial cooling loops, and in industrial waste treatment facilities, because of its selectivity towards specific environmentally-objectionable waste materials, including phenols, sulfides, cyanides, thiosulfates, and mercaptans. In addition, chlorine dioxide is being used in the oil and gas industry for downhole applications as a well stimulation enhancement additive.
Unlike chlorine, chlorine dioxide remains a gas when dissolved in aqueous solutions and does not ionize to form weak acids. This property is at least partly responsible for the biocidal effectiveness of chlorine dioxide over a wide pH range, and makes it a logical choice for systems that operate at alkaline pHs or that have poor pH control. Moreover, chlorine dioxide is a highly effective microbiocide at concentrations as low as 0.1 parts per million (ppm) over a wide pH range.
The biocidal activity of chlorine dioxide is believed to be due to its ability to penetrate bacterial cell walls and react with essential amino acids within the cell cytoplasm to disrupt cell metabolism. This mechanism is more efficient than other oxidizers that “burn” on contact and is highly effective against legionella, algae and amoebal cysts, giardia cysts, coliforms, salmonella, shigella, and cryptosporidium.
Unfortunately, chlorine dioxide can become unstable and hazardous under certain temperature and pressure conditions. Although this is only an issue of concern for solutions of relatively high concentration, its shipment, at any concentration, is banned. It is for this reason that chlorine dioxide is always generated on-site, at the point of use, usually from a metal chlorate or metal chlorite as an aqueous solution. For example, a metal chlorite solution mixed with a strong acid can be used to generate chlorine dioxide in situ.
Electrochemical processes provide a means for generating chlorine dioxide for point of use applications. For example, U.S. Pat. No. 5,419,816 to Sampson et al. describes a packed bed ion exchange electrolytic system and process for oxidizing species in dilute aqueous solutions by passing the species through an electrolytic reactor packed with a monobed of modified cation exchange material. A similar electrolytic process is described in U.S. Pat. No. 5,609,742 to Sampson et al. for reducing species using a monobed of modified anion exchange.
One difficulty with electrochemical processes is that it can be difficult to control the generation of undesirable species. For example, there are many electrochemical reactions that can occur at the anode. Within a potential range of 0.90 to 2.10 volts, at least eight different reactions are thermodynamically possible, producing products such as chlorate (ClO3−), perchlorate (ClO4−), chlorous acid (HClO2), oxygen (O2), hydrogen peroxide (H2O2) and ozone (O3). It is highly desirable and a significant commercial advantage for an apparatus to allow for careful control of the products generated to achieve high yield efficiency.
Chlorine dioxide has also been produced from a chlorine dioxide precursor solution by contacting the precursor solution with a catalyst (e.g., catalysts containing a metal such as those catalysts described for example in U.S. Pat. No. 5,008,096) in the absence of an electrical field or electrochemical cell. However, known catalytic processes have the disadvantage of becoming greatly deactivated within a matter of days. Moreover, it has been found that the support materials for the catalytic sites tend to quickly degrade due to the oxidizing nature of chlorine dioxide. Still further, the use of catalyst materials in packed columns or beds for generating chlorine dioxide has been found to cause a significant pressure drop across the column or form channels within the column that results in a significant decrease in conversion efficiency from the chlorine dioxide precursor to chlorine dioxide. It is also noted that catalyst materials are relatively expensive and can add significant cost to an apparatus employing these materials.